1. Field of the Invention
The present invention relates to an active matrix liquid crystal display device, more specifically, to an active matrix liquid crystal device having improved operation speed, and a method for driving the same.
2. Description of the Related Art
Conventionally, the CRT (cathode ray tube) display is a most commonly used display device. Since the CRT display uses a vacuum glass tube and accelerates electrons by a high voltage, it has such problems as a large capacity, heavy weight, and a large power consumption. Thus, flat panel display devices using plasma or a liquid crystal are now under development.
The liquid crystal display device performs on/off display, i.e., light and shade display by controlling the polarization of light, a transmission light quantity, or a scattering light quantity utilizing the fact that a liquid crystal material has different dielectric constants in directions parallel with and perpendicular to the molecular axis. Commonly used liquid crystal materials is TN (twisted nematic) liquid crystal, STN (super twisted nematic) liquid crystal, ferroelectric liquid crystal and the like.
Particularly in recent years, among various types of liquid crystal display devices, the active matrix liquid crystal display device has come to be used widely.
FIG. 10 shows a conventional active matrix liquid crystal display device. In this active matrix liquid crystal display device, signal lines 1001-1003 and scanning lines 1004-1006 are provided in a matrix form and thin film transistors (TFTs) 1007-1010 are arranged at intersection of those lines. Source electrodes of the TFTs 1007-1010 are connected to the signal lines 1001-1003 and gate electrodes of the TFTs 1007-1010 are connected to the scanning lines 1004-1006. Drain electrodes of the TFTs 1007-1010 are connected to retaining capacitors 1016-1019 and pixel electrodes (not shown) provided for respective liquid crystal cells 1012-1015 of pixel regions.
There are two methods of forming peripheral driver circuits for the pixels. In a first method, the peripheral driver circuits are formed using single crystalline silicon transistor integrated circuits. In a second method, they are formed by polysilicon TFTs and provided together with an active matrix on the same glass substrate. In the first method, the driver circuits are connected to the active matrix by TAB (tape automated bonding) or COG (chip on glass). In the second method, the driver circuits are connected to the active matrix via metal wirings on the substrate, rather than TAB or COG.
FIG. 11 shows a conventional matrix panel having peripheral circuits and a pixel matrix. Pixels of a pixel matrix 1101 are connected to a signal line driving circuit 1104 and a scanning line driving circuit 1105 via signal lines 1102 and scanning lines 1103, respectively.
FIGS. 12A-12C show waveforms of voltages applied to the electrodes of a TFT. In FIG. 12A, an electric signal V.sub.s is applied to the source electrode of the TFT via the signal line. In FIG. 12B, an electric signal V.sub.G is applied to the gate electrode. As a result, a voltage V.sub.D in FIG. 12C produces at the drain electrode.
In an N-channel type TFT, when a high (positive) voltage is applied to the gate electrode, the TFT turns on and the source and drain voltages are made equal to each other. As a result, the voltage on the signal line is stored in a retaining capacitor. Then when a low (negative) voltage is applied to the gate electrode, the TFT turns off and the source and drain electrodes are insulated from each other to obtain an open state. As a result, the voltage of the retaining capacitor is stored until the TFT turns on next time, which causes new writing.
A difference between voltages of the opposed electrode and the pixel electrode is applied to each of the liquid crystal cells 1012-1015 (see FIG. 10) interposed between those electrodes. The light polarizing characteristic of the liquid crystal cell is changed in accordance with the difference voltage. By passing through a polarizing plate, a variation of transmittance is ultimately obtained, thereby providing light and shade display.
The conventional active matrix liquid crystal display device mainly uses the TN liquid crystal because of its low price and ease of orientation control. With passing through a polarizing plate, the TN liquid crystal has a transmittance-applied voltage (V) characteristic of FIG. 13. By virtue of a relatively gentle slope, this transmittance-applied voltage characteristic curve enables gradational display with control by the applied voltage. However, the TN liquid crystal is associated with slow response with respect to the applied voltage. In general, in the TN liquid crystal, there occurs a response delay of 10 ms to several 10 ms when the gradation level is changed from black to white, or vice versa (see FIG. 14).
In one display pixel, a gradation level is changed from black to white, it is observed that the gradation level of a central portion of the display pixel is first changed and the gradation level of its peripheral portion is changed with a delay. This results from a phenomenon that the central portion and the peripheral portion of a liquid crystal in a pixel region have a difference in a response time when an electric field is applied.
To explain this phenomenon, FIGS. 15A and 15B show states of liquid crystal molecules 1509 and electric lines of force 1510 in the conventional active matrix liquid crystal display device when a voltage is applied to the liquid crystal. In FIG. 15A, a liquid crystal cell has a pair of glass substrates 1501 and 1502 and a pair of transparent electrodes 1507 and 1508, and liquid crystal molecules are aligned by applying an electric field between the transparent electrodes 1507 and 1508. In FIG. 15B, electric lines of force between the transparent electrodes 1507 and 1508 in a case of FIG. 15A is shown. By interposing the configuration of FIG. 15A between a pair of polarizing plates, a simplest form of liquid crystal display device can be obtained. An orientation (alignment) film and switching TFTs, which actually exist in addition to the components shown, are omitted from FIGS. 15A and 15B.
When an electric field is applied by the pixel electrodes 1507 and 1508, the liquid crystal molecules 1509 change their orientations so that they become parallel with the electric field uniformly. Thus, a polarization state of light passing through the liquid crystal is changed. In this state, in a peripheral portion of the pixel, the liquid crystal molecules 1509 on the pixel side of a boundary surface 1500 operate to change their orientations while those on the opposite side of the surface 1500 tend to keep their orientations. Thus, a liquid crystal in a pixel side region close to the boundary surface 1500 has slow response speed than that in a central region of the pixel.
Although the conventional active matrix liquid crystal display device can display a still (static) image with image quality equivalent to or better than that of the CRT display, it cannot display a moving (dynamic) image with image quality equivalent to that of the CRT display due to the above response delay of the liquid crystal. This problem is actually observed as an unnatural display at the occurrence of a fast hue variation and slow movement in displaying a moving picture.